Al Bakri et al

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Journal of Engineering and Technology Research Vol. 3(1), pp. 1-4, January 2011Available online at http:// www.academicjournals.org/JETRISSN 2006-9790 2011 Academic Journals

Review

Review on fly ash-based geopolymer concrete withoutPortland Cement

Mohd Mustafa Al Bakri1*, H. Mohammed2, H. Kamarudin1, I. Khairul Niza3 and Y. Zarina1

1School of Material Engineering, Universiti Malaysia Perlis (UniMAP), P. O. Box 77, d/a Pejabat, Pos Besar, 01007

Kangar, Perlis, Malaysia.2King Abdul Aziz City Science and Technology (KACST), P. O. Box 6086, Riyadh 11442, Kingdom of Saudi Arabia.3School of Environmental Engineering, Universiti Malaysia Perlis (UniMAP), P. O. Box 77, d/a Pejabat, Pos Besar,

01007 Kangar, Perlis, Malaysia.

Accepted 19 November, 2010

The consumption of Ordinary Portland Cement (OPC) caused pollution to the environment due to theemission of CO2. As such, alternative material had been introduced to replace OPC in the concrete. Flyash is a by-product from the coal industry, which is widely available in the world. Moreover, the use offly ash is more environmental friendly and save cost compared to OPC. Fly ash is rich in silicate andalumina, hence it reacts with alkaline solution to produce aluminosilicate gel that binds the aggregateto produce a good concrete. The compressive strength increases with the increasing of fly ash finenessand thus the reduction in porosity can be obtained. Fly ash based geopolymer also provided betterresistance against aggressive environment and elevated temperature compared to normal concrete. Asa conclusion, the properties of fly ash-based geopolymer are enhanced with few factors that influenceits performance.

Key words:Fly ash-based geopolymer, alkaline solution, compressive strength.

INTRODUCTION

Ordinary Portland Cement (OPC) becomes an importantmaterial in the production of concrete which act as itsbinder to bind all the aggregate together. However, theutilization of cement causes pollution to the environmentand reduction of raw material (limestone). Themanufacturing of OPC requires the burning of largequantities of fuel and decomposition of limestone,resulting in significant emissions of carbon dioxide (Kongand Sanjayan, 2008). As such, geopolymer concrete had

been introduced to reduce the above problem.Geopolymer concrete also showed good properties suchas high compressive strength, low creep, good acidresistance and low shrinkage (Lodeiro et al., 2007). Therole of binder in geopolymer concrete is replaced by flyash which also possess pozzolanic properties as OPCand rich with alumina and silicate. Fly ash is residue from

*Corresponding author. E-mail: [email protected].

the combustion of coal which is widely availableworldwide and lead to waste management proposalHence, fly ash-based geopolymer concrete is a goodalternative to overcome the abundant of fly ash. In flyash-based geopolymer concrete, the silica and thealumina present in the source materials are first inducedby alkaline activators to form a gel known asaluminosilicate.

This gel binds the loose aggregates and other un

reacted materials in the mixture to form the geopolymeconcrete (Wallah, 2009). Besides that, the reaction alsodepends on a few parameters such as size ofaggregates, chemical composition of fly ash, amount ovitreous phase in fly ash, nature, concentration and pH oactivators. The curing process of geopolymer concreteplay shows a great influence on the development omicrostructure, and subsequently on the mechanicacharacteristics of geopolymer (Komljenovic et al., 2010).

This paper summarizes the properties of fly ash-basedgeopolymer which make it better compared to normaconcrete.


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ALKALINE SOLUTIONS

The common materials used as alkaline solution inproducing fly ash-based geopolymer are sodium silicateand potassium hydroxide. Usually either of this materialwas mixed with sodium hydroxide to produce the alkaline

solution and the molarity (M) of alkaline solution is 7 to 10M. The alkaline solution was prepared a day before it ismixed with fly ash. Then, the materials are mixedtogether with fine aggregate and coarse aggregate toform concrete and curing process been done.

Chindaprasirt et al. (2007) found that, to produce higherstrength geopolymer, the optimum sodium silicate tosodium hydroxide ratio was in range of 0.67 to 1.00.Meanwhile, the concentration of NaOH between 10 and20 M give small effect on the strength.

CURING PROCESS

Setting time of geopolymer depend on many factors suchas composition of alkaline solution and ratio of alkalineliquid to fly ash by mass. However, the curingtemperature is the most important factor for geopolymer.As the curing temperature increases, the setting time ofconcrete decreases (Chanh et al., 2008). During curingprocess, the geopolymer concrete experiencepolymerization process. Due to the increasing oftemperature, polymerization become more rapid and theconcrete can gain 70% of it strength within 3 to 4 h ofcuring (Kong and Sanjayan, 2008).

PROPERTIES OF GEOPOLYMER

Workability of fresh geopolymer

Water also play important role in geopolymer concrete asmuch as normal concrete. The used of water ingeopolymer is to improve the workability, but it willincrease the porosity in concrete due to the evaporationof water during curing process at elevated temperature(Sathia et al., 2008).

Chindaprasirt et al. (2007) discovered an increase insodium hydroxide and sodium silicate concentration willreduce the flow of mortar. The workable flow ofgeopolymer mortar was in the range of 110 5 to 135 5%.

To improve the workability of mortar, superplasticiser orextra water can be added. However, the use ofsuperplasticiser had an adverse effect on the strength ofgeopolymer. As such, extra water gives higher strengththan addition of superplasticiser.

Compressive strength

Compressive strength is an essential property for allconcrete where it also depends on curing time and curingtemperature. When the curing time and temperature

increase, the compressive strength also increases. Withcuring temperature in range of 60 to 90C, within time in24 to 72 h, the compressive strength of concrete can beobtained about 400 to 500 kg/cm

2(Chanh et al., 2008). In

addition, the compressive strength of geopolymers alsomainly depended on the content of fly ash fine particles

(smaller than 43 m).The compressive strength was increase when the finesof fly ash increase. Hence the nature and theconcentration of the activators were dominant factors inthe reaction of alkali activation. The highest compressivestrength was obtained using a solution of sodium silicateas an activator (n = 1.5; 10% Na2O). Sodium silicate isthe most suitable as alkaline activator because it containsdissolved and partially polymerized silicon which reactseasily, incorporates into the reaction products andsignificantly contributes to improving the mortacharacteristics (Komljenovic et al., 2010).

Resistance against aggressive environment

Fly ash-based geopolymer had been proved by manystudies to provide better resistance against aggressiveenvironment. As such, this advantage can be used toconstruct structure that exposed to marine environmen(Chanh et al., 2008).

The exposure of geopolymer in acid solution showsthat the weight loss due to the exposure is only 0.5%compared to normal concrete when immersed in 3% acidsulfuric (Sathia et al., 2008).

According to Bakharev (2005) in acidic exposure, high-performance geopolymer materials deteriorate with the

formation of fissures in amorphous polymer matrix, whilelow-performance geopolymers deteriorate throughcrystallisation of zeolites and formation of fragile grainystructures. The formation of aluminosilicate gel isimportant to determine the stability of geopolymer. Morecrystalline geopolymer material prepared with sodiumhydroxide was more stable in the aggressive environmenof sulfuric and acetic acid solutions than amorphousgeopolymers prepared with the sodium silicate activator.

Thokchom et al. (2009) expose the geopolymer mortarin 10% sulfuric acid and found specimens still intact anddid not show any recognizable change in colour after 18weeks. When observed under optical microscope, the

expose surface reveled a corroded structure and iprogress with time of exposure. In addition, the weightloss results obtained in this study showed betteperformance than OPC and the specimens with higheralkali content were observed to lose more weight thanspecimens with lower alkali content. At 18 weeks, thespecimens were fully dealkalized by the sulfuric acid as inFigure 1, but it still had substantial residual compressivestrength to prove the higher resistance against acidexposure (Thokchom et al., 2009). Thokchom et al(2009) investigated the effect of sodium oxide content ondurability of geopolymerinsulphuricacid.Specimens with


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Figure 1. Residual alkalinity in 10% acid sulfuric solution.

Figure 2. Fly ash-based geopolymer specimens with white deposit after 24 weeks exposure.

varying alkali content showed varying degree ofdeterioration when exposed to sulphuric acid. Besidethat, the specimens showed no visible sign of structuraldisintegration but under optical microscope observation,the surface deterioration was clearly visible and itappeared more severe in specimen with less alkalicontent. The specimen with maximum alkali contentexperienced more reduction in weight compared tominimum alkali content. One the other hand, specimenwith minimum alkali content loss more strength. Hence,specimen with higher alkali content performed much

better than those with lower alkali content in terms ofresidual compressive strength.

When the fly ash-based geopolymer was exposed tomagnesium sulphate environment for 24 weeks, the whitedeposit were observed on the surface on the specimensas shown in Figure 2. Nevertheless, the observationsunder optical microscope shown, only fine cracks wereseen on a few specimens but there are no visible cracksobserved on the specimen. The gain of specimen weightwhen immersed in magnesium sulphate depends of thesodium oxide content produced from alkaline activator.

The maximum weight gain was produced by specimenwith minimum content of sodium oxide. Even so, all thespecimens only recorded a small gain weight comparedto normal concrete (Thokchom et al., 2010). Lodeiro et al(2007) discovered that geopolymer are less prone toformation of alkali silica gel.

BEHAVIOUR OF GEOPOLYMER AT ELEVATEDTEMPERATURE

Fire resistance of concrete was other properties that arealways considered due to the safety of user. Kong andSanjayan (2008) said that the ratio of fly ash to alkalinesolution influence the general strength and fire resistanceof geopolymer. It was found that the fly ash-basedgeopolymer displayed increase in strength aftetemperature exposure.

Kong and Sanjayan (2010) observed the behavior ogeopolymer concrete under elevated temperatureaffected by the size of aggregates. The aggregate withsmaller sized (


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extensive cracking of geopolymer but the largeraggregate (>10 mm) were more stable. In addition, thethermal incompatibility between the geopolymer matrixand its aggregate components was the most likely causeof strength loss in geopolymer concrete specimens atelevated temperatures. It can be proved by comparison

between geopolymer concretes made two differentaggregates with distinctly different thermal expansioncharacteristics. The geopolymer concrete with greaterincompatibility led to higher strength loss during elevatedtemperature. Thus, the expansion of aggregate withrespect to temperature was a factor that controls theperformance of geopolymer.

Bakharev (2006) observed the thermal stability of thegeopolymer materials prepared with sodium containingactivators was rather low and significant changes in themicrostructure occurred. At 800C, the strength of theconcrete was reduced due to the increase in the averagepore size where amorphous structures were replaced bythe crystalline Na-feldspars. The reverse situation wasobserved when potassium silicate was used as activatorsbecause it can remain mostly amorphous up to 1200C.After firing these materials, it reduced average pore sizeand improved compressive strength of geopolymer. Flyash based geopolymer prepared using class F fly ashwith sodium and potassium silicate show high shrinkageas well as large changes in compressive strength withincreasing fired temperature in the range 800 to 1200C(Bakharev, 2006).

CONCLUSION

Fly ash-based geopolymer is better than normal concretein many aspects such as compressive strength, exposureto aggressive environment, workability and exposure tohigh temperature.

REFERENCES

Bakharev T (2006). Thermal behaviour of geopolymers prepared usingclass F fly ash and elevated temperature curing. Cement ConcreteRes., 36: 1134-1147.

Bakharev T (2005). Resistance of geopolymer materials to acid attackCement Concrete Res.. 35: 658-670.

Chindaprasirt P, Chareerat T, Siricicatnanon V (2007). Workability and

strength of coarse high calcium fly ash geopolymer. CemenConcrete Compos., 29: 224-229.

Chanh NV, Trung BD, Tuan DV (2008). Recent research geopolymeconcrete. The 3

rdACF International Coference ACF/VCA.

Kong DLY, Sanjayan JG (2008). Damage behavior of geopolymecomposites exposed to elevated temperature. Cement ConcreteCompos., 30: 986-991.

Kong DLY, Sanjayan JG (2010). Effect of elevated temperaturesgeopolymer paste, mortar and concrete. Cement Concrete Res., 40334-339.

Komljenovic M, Bascarevic Z, Bradic V (2010). Mechanical andmicrostructural properties of alkali-activated fly ash geopolymer. JHazardous Mater., 181: 35-42.

Lodeiro G, Palomo A, Jimnez F (2007). Alkali-aggregate reaction inactivated fly ash system. Cement Concrete Res., 37: 175-183.

Sathia R, Babu KG, Santhanam M (2008). Durability study of lowcalcium fly ash geopolymer concrete. The 3

rdACF Internationa

Coference ACF/VCA.Thokchom S, Ghosh P, Ghosh S (2010). Performance of Fly ash Based

Geopolymer Mortars in Sulphate Solution. J. Eng. Tech. Rev., 3(1)36-40.

Wallah SE (2009). Drying Shrinkage of Heat-Cured Fly Ash-BasedGeopolymer Concrete. Modern Appl. Sci., 3: 12.

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